1. Field of the Invention
The present invention relates generally to a driving method and a driving device for a motor, and more particularly, to a driving method and a driving device for a concentrated winding synchronous motor.
2. Description of the Related Art
FIGS. 5A, 5B and 5C illustrate partially sectional views of a concentrated winding synchronous motor. Referring to the drawings, a rotor 32 shown in FIG. 5B has been rotated counterclockwise at 7.5 from the state of the rotor shown in FIG. 5A as a reference, and the rotor 32 shown in FIG. 5C has been rotated counterclockwise at 15 from the reference state shown in FIG. 5A.
In the concentrated winding synchronous motor shown in the drawings, a stator 35 having a plurality of teeth 34a through 34f is fitted inside a cylindrical case 30. In the case of the motor shown in the drawings, the coil of the stator 35 is of the concentrated winding type, and each of the teeth 34 is wound with a coil of either phase U, V, or W.
The rotor 32 is pivotally supported inside the stator 35. Radially magnetized permanent magnets are attached to the outer surface of the rotor 32. These permanent magnets function as magnetic poles 36a through 36d of the rotor 32. In this type of motor, three phase alternating current is supplied to the coils of the teeth 34 to form a rotating magnetic field in the stator 35, and the repeated cycle of attraction and repulsion between the respective magnetic poles 36 and the teeth 34 serves to drive the rotor 32 at a predetermined torque.
Adoption of such concentrated winding can simplify the manufacturing process of the synchronous motor owing to the easy installation of the coil compared with the distributed winding (wrapping) in which the coil is applied to wind about one or more teeth 34.
However, applying the coil of concentrated winding type to the teeth 34 may fail to efficiently obtain the required torque.
FIG. 6 illustrates how the aforementioned problem arises. The upper side of FIG. 6A shows the portion interposed between the magnetic poles 36b, 36c and the teeth 34b through 34e opposite thereto in a rotating direction in FIG. 5A. The lower side of FIG. 6A shows the magnetic field formed by the stator 35 at the position of the rotor 32. FIGS. 6B and 6C likewise show each of the portions interposed between the magnetic poles 36b, 36c and the teeth 34b through 34e opposite thereto illustrated in FIGS. 5B and 5C, and the magnetic field formed by the stator 35 at the position of the rotor 32.
As indicated in FIG. 6A, the S pole magnetic field (inward magnetic field) formed by the tooth 34b and the N pole magnetic field (outward magnetic field) formed by the tooth 34c coexist separately left and right on the surface facing the magnetic pole 36b. Similarly the N pole magnetic field formed by the tooth 34c and the S pole magnetic field formed by the tooth 34e coexist left and right on the surface facing the magnetic pole 36c. Therefore, when the rotor 32 is at the position illustrated in FIG. 6A or FIG. 5A, the magnetic poles 36b and 36c are attracted by the respective destination teeth 34c and 34e as well as being repelled by the respective teeth 34b and 34c behind thereof, thereby enabling the rotor 32 to generate torque efficiently.
In the state shown in FIG. 6C, the magnetic poles 36b and 36c are likewise attracted by the respective destination teeth 34d and 34e as well as being repelled by the respective teeth 34b and 34d behind thereof, thereby enabling the rotor 32 to generate the torque efficiently.
However, a state shown in FIG. 6B exists in the process proceeding from the state of FIG. 6A to that of FIG. 6C. That is, since the current supplied to each phase has a sinusoidal wave, the generated N pole magnetic field opposing the magnetic pole 36b becomes relatively weak compared with the state shown in FIG. 6A, and the generated N magnetic field opposing the magnetic pole 36c becomes relatively strong as compared with the state shown in FIG. 6A. In this state, a part of the magnetic fields formed on the surface facing the magnetic poles 36b and 36c may fail to contribute to generation of the torque, and act to reduce the torque generated. In the case of the magnetic pole 36b, as shown by arrow A in FIG. 6B, a portion of N pole magnetic field generated by the tooth 34c exists behind a centerline 38b of the magnetic pole 36b, that is, at the side reverse to the rotating direction. Therefore the magnetic pole 36b is pulled back in a direction reverse to the rotating direction. The aforementioned phenomenon applies to the magnetic pole 36c. As indicated by arrow B in FIG. 6B, a portion of the N pole magnetic field generated by the tooth 34d exists at a forward side, in the rotating direction, of a centerline 38c of the magnetic pole 36c. This may push back the magnetic pole 36c in a direction reverse to the rotating direction.
Accordingly, when adopting the concentrated winding for the synchronous motor coil, depending on the position of the rotating rotor 32, a portion of each phase current supplied to the coil of the stator 35 of the synchronous motor may serve to impede generation of torque. As a result, the rotating torque is not efficiently obtained.